S-15 4x4 stalling

I posted in GMC but so far no one has looked at it. I recently bought a 1986 GMC S-15 4x4 for 1400$. I had bought it off a friend of mine who said it has a stalling problem and that its a 80 dollar fix. I took it to a mechanic and he said it was the o2 sensor so i had it chagned today and it still stalls after runnning. It has a bigger cam put into it for more power its a 2.8 V6 engine. the service engine light is on. and how do you reset the computer?

Another thing is that it will run if you always have your foot on the gas pedel

last time a full tuneup was done... if its a carb that includes needles n' points

it was about 8 months ago, They replaced plugs wires rotor, disteributor, oil, brakes, the whole 9 yards. its TBI fuel injected

was the fuel filter done? and when was the last time the pump was changed/ sock cleaned?

I got the truck into my school automotive class and they said it was a vaccum leak to the MAP sensor. They replaced the sensor before i had gotten the truck, but they didnt reconncet the vaccm hose. I also got a nice surprize of the truck being on my front lawn after school. and my lawn is a good 4 ft off the street

Ok Ive got a question. Does the ECM regulate gas flow into the engine, or is that a totally different thing, because now, the truck when i first start it runs rough and evens out eventually, but i just jump in a go, i run the risk of it stalling when i downshift, and i know that I'm shifting corectly. I had the injectors cleaned and the fuel system cleaned from tank to injectors, filter was done about 3 months ago new air cleaner and it had a total tune up about 5 months ago. this is the only serious problem I have with it besides my dash is messed up and the spedo can only go up to 60 km before getting stuck and wont come back down even if im stopped or my signal lights are turning on when i turn on my lights or brights.....